EDIT: Second medical use patents are enforced as products for a give use. So the alleged infringer must have intention the product is used in a certain way to be found liable. What follows is a possibly-off-topic-but-hopefully-useful discussion of how intention might be considered in a second medical use situation. Note that the original question specifically asked about labeling and marketing. So this might seem off topic but actually gets to the heart of what the question is about, even if that's not apparent at first glance.
Excellent question. As pointed out, this has been the subject of some interesting court cases in recent years.
The interplay with the regulation of medicines is important here. In most (perhaps all?) countries a compound cannot be sold as being for the treatment of a disease (i.e., as a medicine) without an appropriate marketing authorization (MA) from the relevant regulator. The MA is for a particular condition or limited set of conditions. This allows you to put on the label (a.k.a., the SmPC or patient information leaflet) what the compound is for, medicinally, and to talk about it as a medicine in your marketing materials, etc.
If you come to know that the compound can be used in a new medical use, then you can't just start selling it for that use. You'd have to have an appropriate MA to update the label to include the new condition, or the regulator would come down on you like a tonne of bricks. If you have the right MA and you advertise the compound as being for the new medical use, then it's pretty clear what your intentions are. The holder of a second medical use patent covering the new use would therefore be able to say that you were selling the compound for the use in their patent: you've pretty much been forced to admit it to avoid heat from the regulator.
But you'd still be free to sell the compound for the use in your original MA, i.e., for the treatment of the old condition. You just can't say it's for treatment of the new condition. You might even manufacture the same product with multiple different versions of the label for different territories, which might be as simple as a sticker that goes on the boxes/bottles that are shipped to a territory where the second medical use is patented. This process is called skinny labeling or label carve-out.
However, where this gets complicated is that although you might never say your product is for the new use, it's possible that doctors will prescribe your product for the new use anyway. This is called off-label prescribing and is a routine thing doctors do.
SECOND EDIT: What if the alleged infringer knows that its product is being used off-label. What if they tacitly encourage it? Are they liable now? This is a complex infringement question that has been reviewed by courts across Europe.
So the position that some courts in Europe have reached is that the manufacturer has to do something to avoid off-label use in order to avoid infringement. Exactly what will depend on the territory.
It should be noted that the Finnegan article linked to in an earlier answer is a little out of date as the UK Supreme Court has since issued a decision on this topic (summary here).